Chemistry Reference
In-Depth Information
factors include doping type and concentration, potential, and illumination direction and
determine the overall range of pore diameters. The second group of factors include
current density, HF concentration, illumination frequency, and intensity and determine
the variation of pore diameter and interpore spacing. Eventually, the change of pore
diameter, pore wall thickness, and transition from PS formation to electropolishing on
silicon are determined by the change of the distributions of the reactions and their rates
along the pore bottom by changing the shape of the curved pore bottom. Except for the
macro PS on lowly doped
p-
Si
,
both pore diameter and wall thickness are largely deter-
mined by the width of the space charge layer. Pore diameter is determined by the radius
of curvature which is on the same order of magnitude as the width of the space charge
layer. Wall thickness is generally less than twice the space charge layer thickness so that
the walls are depleted of carriers because of the overlapping of the two space charge
layers. If the wall thickness is larger than twice the space charge layer, the walls are not
depleted of carriers and dissolution can still occur and new pores can form along the wall
and propagate into the wall. Under certain conditions such as back-side illumination and
surface patterning, pore walls can be much larger than twice the space charge layer
because of the large relative depletion of the carriers in the walls compared with carrier
concentration at the pore tips as discussed in the preceding section.
The actual wall thickness depends on the relative dissolution rates between the
and If is com-
parable to significant dissolution occurs at the edge of the pore bottom before the
pore tip propagates a distance away. This will result in a thin wall, or even no wall at
all when equals On the other hand, if is very small compared to the pore tip
will propagate relatively fast so that, before much dissolution occurs on the edge of the
pore bottom, the edge has already moved into the wall region where the current is very
small and the dissolution is virtually stopped. This will generate relatively thick walls
(compared to pore size).
edge of a pore bottom (see Fig. 8.68) and the tip, that is, between
